DECServer 90L+ startup help

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sun Dec 28 20:34:05 2003

> > Have you got a proper thinwire segment between all of them? The one thing
>
> Thinwire/thickwire = does this equal x-length of good 50 ohm coax with
> BNCs on either end? If so - thats covered. What I'm *not* doing is

That should be OK for the cable.

> terminating - I didn't figure I'd have to with each connector on the
> servers joined by a single run of coax. I have the BNC Ts - should each
> end also need 50 Ohms...? Mongo not Grok LAN voodoo...

YES. You _must_ terminate both ends of the thinwire segment, no matter
how short it is. This is due to the design of the ethernet transceiver,
the transmit part of which is a current source that develops a voltage
across the terminator resistors. If you don't terminate properly, then
the votlage on the network cable goes too high, which is exactly how a
colision is detected...

Fit the T-pieces and 50 Ohm terminetors (a BNC plug with a 50 ohm -- or
51 ohm -- resistor between pin and case) at each end and try again.

> > that always caught my customers out was the end connections MUST be t-pieces
> > with 50 ohm terminators, even on the servers. If you've got a DS90 on its
> > own it must have a t-piece with 2 terminators on its thinwire port,
>
> 2 50 Ohm terms per port? Mongo very confused now... Mongo Least

If you have nothing connected to the thinwire port (no network cable),
then you need to provide a 25 ohm resistance (i.e. 2 50 ohms in parallel
-- the DC condition of a terminated thinwire cable) between the centre
and sheild of the thinwire connector. Otherwise the darn thing fails the
selftest (IIRC) for much the same reason as above.

-tony
Received on Sun Dec 28 2003 - 20:34:05 GMT